An Internet Explorer shell is any computer software that uses the Tridentrendering engine of the Internet Explorerweb browser. Although the term "Trident shell" is probably more accurate for describing these applications (including Internet Explorer itself), the term "Internet Explorer shell", or "IE shell", is in common parlance. This means that these software products are not actually full-fledged web browsers in their own right but are simply an alternate interface for Internet Explorer; they share the same limitations of the Trident engine, typically contain the same bugs as IE browsers based on the same version of Trident, and any security vulnerabilities found in IE will generally apply to these browsers as well. Strictly speaking, programs that use Tasman (layout engine), used in Internet Explorer 5 for Apple Mac, are also IE shells, but, because Internet Explorer for Mac was discontinued in 2003, and Tasman was further developed independent of IE, it tends to be thought of as a separate layout engine.

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These applications supplement some of Internet Explorer's usual user interface components for browsing, adding features such as popup blocking and tabbed browsing. For example, MSN Explorer can be considered an Internet Explorer shell, in that it is essentially an expansion of IE with added MSN-related functionality. A more complete list of Trident-based browsers can be found under the list of web browsers.

Other applications that are not primarily for web browsing, such as Intuit's Quicken and QuickBooks, AOL, Winamp, and RealPlayer, use the rendering engine to provide a limited-functionality "mini" browser within their own user interfaces.

The Trident engine was, until Outlook 2007, also used to render HTML portions of email messages in Microsoft Outlook and Outlook Expressemail clients (Outlook 2007 now uses Microsoft Word to render HTML e-mail). This integration, while convenient, is an often-exploited "back door", since the Internet Explorer components make available more functionality to the HTML code than some feel should be permitted in the context of email messages, and Outlook and Outlook Express have, historically, not done enough to prevent malicious code from taking advantage of that functionality. The latest updates for Outlook Express, which require Windows XP and are distributed with Service Pack 2, are intended to improve this situation. Outlook 2003 already includes many of the updates.

While all of these programs can customize Internet Explorer's user interface and extend the feature set, they cannot modify the rendering engine, and are therefore subject to many of the same benefits and vulnerabilities of IE, including security holes and issues with rendering.

HTA is a method developed by Microsoft to open up HTML files in a window free of navigation and other interface elements.

In addition to programs using Internet Explorer’s rendering engine, there are also programs that add features to Internet Explorer. The methods they use to add features can blur the distinction between a shell, plug-in, or toolbar. Examples include the following:

HttpWatch: A traffic monitoring and debugging tool for Internet Explorer that displays information about headers, cookies, redirection and compression.[5]

IEWatch: Provides diagnostic panes for monitoring HTTP traffic, HTTP search and filtering, HTTP performance charts and the ability to view the DOM and quickly modify the HTML, style sheets and scripts of any web page to test code.[6]

Kimba Kano: Adds a context menu option to do various kinds of web searches for a currently highlighted term.[10]

PageEngage: An HTTP analysis tool which allows a user to view all HTTP headers, provides a script console for fully interacting with the Document Object Model of Internet Explorer, and additionally provides the user the ability to modify the response content before the browser renders it.[11]

WebReplay is an IE plug-in for browser automation and web testing. With WebReplay one can fill web forms and automate web applications along with many other activities.[12]